Optical recording media, such as a CD with a storage capacity of 0.65 GB and a DVD with a storage capacity of 4.7 GB, have become popular as storage media for storing information such as video, audio, and computer data. Also, demand has become high for an optical recording medium with a higher recording density and a higher storage capacity.
One way to improve the recording density of an optical recording medium is to reduce the diameter of a light spot formed on an optical recording medium by an optical pickup used to write and read information onto/from the optical recording medium. The diameter of a light spot can be reduced by increasing the numerical aperture (NA) of an objective lens, which is used to focus light, of an optical pickup or by reducing the wavelength of a light source of an optical pickup.
For example, while a normal pickup for CDs includes an objective lens with an NA of 0.50 and a light source with a wavelength of 785 nm, a normal pickup for DVDs includes an objective lens with an NA of 0.65 and a light source with a wavelength of 660 nm. To further improve the recording density and storage capacity of an optical recording medium, it is necessary to make the NA of an objective lens larger than 0.65 or the wavelength of a light source shorter than 660 nm.
As described in patent document 1, two standards for high-capacity optical recording media and optical information processing apparatuses have been proposed. One of them is the Blu-ray Disc (BD) format that uses a light in the blue wavelength range and an objective lens with an NA of 0.85, and offers a storage capacity of up to 22 GB. The other one of them is the HD-DVD (HD) format that uses a light in the blue wavelength range and an objective lens with an NA of 0.65, and offers a storage capacity of up to 20 GB.
The BD format achieves a higher storage capacity than a conventional DVD format by using a light with a shorter wavelength and an objective lens with a larger NA. The HD format achieves a higher storage capacity than a conventional DVD format without increasing the NA of an objective lens by improving the track recording density through a unique signal processing method and by using a land/groove recording technology.
Both formats use a blue-violet semiconductor laser having an oscillation wavelength of about 405 nm. The substrate thickness of a Blu-ray disk is 0.1 mm and that of an HD-DVD is 0.6 mm.
When developing an optical pickup for recording and/or reproducing information on a Blu-ray disk or an HD-DVD, it is preferable to make the optical pickup compatible with conventional CDs and/or DVDs that have been widely used. Also, assuming both of the BD and HD formats become popular at the same time, an optical pickup is preferably designed to handle all of the BD, HD, DVD, and CD formats.
In this case, an optical pickup is preferably designed to select a light source having a suitable wavelength depending on the type of an optical recording medium, to perform appropriate optical processing on the light emitted from the selected light source, and thereby to correct the spherical aberration caused by the difference in substrate thickness of optical recording media.
Patent document 2 discloses an optical pickup that includes two objective lenses and is capable of recording and/or reproducing information on four different types of optical recording media.
[Patent document 1] Japanese Patent Application Publication No. 2005-339718
[Patent document 2] Japanese Patent Application Publication No. 2005-209299
However, using two objective lenses as disclosed in patent document 2 increases the number of parts in an optical pickup and therefore increases the size and costs of an optical pickup. Also, in the disclosed optical pickup, it is necessary to move the objective lenses according to the type of an optical recording medium. Such a configuration requires a complicated mechanism for an actuator and increases access time for reading/recording information.
Therefore, to reduce the size and costs, an optical pickup is preferably designed to record and/or reproduce information on different types of optical recording media using the same optical system.